Gasoline and distillate fuels are regulated for a variety of reasons including, but not limited to, environmental concerns. For example, the U.S. Environmental Protection Agency (EPA) regulates hazardous air pollutants such as benzene from mobile sources. Specifically, the U.S. EPA has regulated the amount of benzene that gasoline may contain. Environmental concerns related to gasoline and distillate fuels are also addressed by the requirements to use renewable resources in the production of gasoline and distillate fuels.
Benzene occurs naturally in petroleum and is formed when petroleum naphtha is reformed to make high octane gasoline blending components. To reduce the benzene content, refiners have tried reducing the benzene-precursors in the feed to the naphtha reformers. However, when using this approach, benzene is still present in the gasoline blending components at levels that make it difficult to achieve regulated limits.
To reduce the benzene content, refiners have also taken two main approaches to meet the benzene limits by removing benzene in the gasoline blending components. They have (1) extracted the benzene and sold it as a petrochemical, and (2) hydrogenated the benzene in hydrocrackers and/or naphtha hydroisomerization units to form methylcyclopentane, cyclohexane, and mixtures thereof (cycloparaffins). The option to sell benzene is limited to locations where there is a market for this petrochemical. In addition, the price of benzene has decreased as many refiners have tried this solution. Given the limited market and low prices, many refiners have chosen instead to hydrogenate the benzene. However, one problem with hydrogenating the benzene is that the products from hydrogenating benzene are methylcyclopentane and cyclohexane. Both products are fairly volatile compounds and as a result can make meeting vapor pressure limits in gasoline difficult.
Unlike the requirements surrounding benzene, the requirements to incorporate renewable resources into the production of gasoline and distillate fuels are driven not just by environmental concerns but by supply concerns as well. For example, renewable resources can be used to produce ethanol which can then be added to gasoline. However, with the incorporation of ethanol in gasoline, gasoline still needs to comply with other specifications. Specifically, the Reid Vapor Pressure becomes a concern when volatile ethanol is blended into gasoline. As shown in Table 1 below, ASTM D4814 defines six Vapor Pressure/Distillation Classes for gasoline and the choice of the class depends on the location within the United States and the season. Colder climates (E) need more volatile gasoline for smooth starts while warmer climates (AA) need low volatile gasoline to avoid excessive evaporative losses and resulting air pollution.
TABLE 1Vapor Pressure Specifications from ASTM D4814Maximum VaporClassPressure (psia)AA7.8A9.0B10.0C11.5D13.5E15.0
Adding small amounts of ethanol (less than 1%) into gasoline results in an additional 1 psi vapor pressure increase. The response is highly non-linear. The vapor pressure from 10% ethanol will approximately equal the vapor pressure maximum for warm climates. In this case, the vapor pressure of the other 90% must be very low.
As shown in Table 2 below, the products from the hydrogenation of benzene (cyclohexane and methylcyclopentane) have significant vapor pressures that can make blending with ethanol to make a specification fuel difficult, especially in warm climates. In addition to the products from hydrogenation of benzene, isopentane is a typical abundant component of petroleum. It too has a high vapor pressure and blending it with ethanol in gasoline is difficult. If pentanes cannot be blended into gasoline, alternative uses must be found, and those uses are typically low-value uses such as a feedstock for petrochemical production, for hydrogen production, or as a refinery fuel. Since refinery fuel is used as the feedstock for hydrogen production, both of the latter two options are low value options. In many locations, the option of selling pentanes into petrochemical use is not possible because of the distance from the market. In these cases, pentanes will be used as low-value refinery fuel.
TABLE 2Properties of Relevant Gasoline HydrocarbonsRONMONRVPBenzene98.090.03.224Cyclohexane8377.13.263Methylcyclopentane91.380.04.503Methylcyclohexane74.871.11.6081,1-Dimethylcyclopentane92.389.32.561,1-Dimethylcyclohexane87.385.80.821-Methyl, 1-ethylcyclopentane100.090.00.721-Methyl, 1-ethylcyclohexane76.768.7~0.5Isopentane92.090.320.443,3-Dimethylpentane86.680.8~22,3-Dimethylpentane91.188.52.35Ethanol112-120*95-106*50-100*(Above blending values in Table 2 are from the following: http://www.ec.gc.ca/cleanair-airpur/CAOL/transport/publications/ethgas/ethgas4.htm.)
Due to the need to lower the content of benzene in gasoline and distillate fuels and due to the need to incorporate renewable resources into the production of gasoline and distillate fuels, a way to incorporate both objectives would be useful. What is needed is a way to use biologically-derived ethanol in gasoline while simultaneously enabling the blending of products from the saturation of benzene. What is also needed is a way to use this ethanol with other volatile compounds from petroleum such as isopentane.